Machine efficiency is an increasingly important consideration for large industrial, construction, and off-highway machines. This is due in part to ever-increasing fuel prices, but is also caused to some extent by increasingly stringent pollution limitations. One technology used to improve machine efficiency while also providing an improved operator experience is the CVT or Continuously Variable Transmission. While there are many types of CVTs, the type most often found on larger machines is the parallel path variator transmission or hydrostatic parallel path transmission.
This type of transmission employs a gear train that receives an input from the engine as well as from a hydraulic motor with continuously variable output. By smoothly varying the hydraulic motor speed, the final output of the gear train may be continuously varied over a wide range of speeds and/or torque values. As useful as this system is, there are still certain drawbacks that remain to be addressed.
Of interest here, CVT transmissions of the type described above have many sources of nonlinearity, and can thus be difficult to control accurately. Moreover, the failure to control the transmission with sufficient accuracy may result not only in an unpleasant operator experience, but may also result in damage to the machine. For example, a CVT used in an off-highway machine application can experience mechanical damage if the engine and/or power train exceeds predetermined over-speed limits during vehicle deceleration. This situation can arise when an operator decelerates the machine using the engine and transmission rather than by using the service brakes. On the other end of the speed range, under-speed problems can occur in a wheel loader application, for example, when the bucket enters a material pile suddenly, causing the engine to lug down.
As noted above, the nonlinearity and complexity of CVT control systems can render overshoot and undershoot control difficult. In some cases, the over speed control logic must change substantially during directional shifts, e.g., the system dynamics change significantly between 2 to 2, 3 to 3, and high speed directional shifts. In order to deal with the nonlinearities of such a system error based gain scheduling techniques have been applied to the standard PID control. However, it is remains challenging in reality to meet all performance requirements with an error based gain scheduling PID controller due to the complexity of the non-linear system dynamics. Thus, in the inventors' observation, a new system of CVT control is needed for replacement of the standard error based gain scheduling PID control in off-highway applications.